This invention relates to the polymerization of mono-1-olefins, such as ethylene, using a catalyst system comprising a supported chromium oxide catalyst and an alkyl lithium cocatalyst.
It is well known that mono-1-olefins, such as ethylene, can be polymerized with catalyst systems employing vanadium, chromium or other metals an inorganic oxide supports such as alumina, silica, aluminophosphate, titania, zirconia, magnesia and other refractory metals. These catalyst systems can be used to prepare olefin polymers in a hydrocarbon solution to give a product having excellent characteristics from many standpoints. Supported chromium oxide catalyst systems also can be used to prepare olefin polymers in slurry polymerization systems wherein the polymer is produced in the form of small particles of solid material suspended in a diluent. This process, frequently referred to as a particle-form process, has an advantage of being less complex.
However, certain control operations which are easily carried out in a solution process are considerably more difficult in a particle-form, or slurry, process. For instance, in a solution process, polymer melt index and/or high load melt index can be varied by changing reactor temperature. Usually a lower melt index can be obtained by decreasing reactor temperature. However, in a slurry process this technique is inherently limited since efforts to decrease melt index and/or high load melt index to any appreciable extent by decreasing reactor temperature can decrease production rates and decrease catalyst activity. Also, fluff bulk density is related to reactor temperature, and lowering the reactor temperature excessively can result in poor bulk density and a high level of fines. Polymers having a high fluff bulk density and a low melt index and/or high load melt index are useful for applications as pipe and drum polymers and/or film polymers. Cr/silica-titania catalysts are preferred for this application, especially when CO treated so that they will produce their own hexene monomer. Unfortunately, however, when such catalysts are used with trialkyl boranes according to prior art, the melt index becomes too high for easy reactor control and good bulk density.
Polymers having a bimodal molecular weight distribution, produced either in the same reactor or by blending two different molecular weight polymers, are especially desirable for use in pipe and drum applications, as well as film applications. Producing such bimodal polymers by adding two catalysts into one reactor can be more economical and practical than blending polymers. But it is difficult to find catalysts capable of producing both extremely high and extremely low molecular weight under the same reactor conditions, and also which incorporate hexene well and poorly respectively. Cr/silica-titania catalysts are particularly good for the high molecular catalyst component when used with a titanium chloride based Ziegler catalyst for the lower molecular weight component. Cr/Silica-titania incorporates hexene very efficiently when CO reduced and when it has a high pore volume. However, such chromium catalysts do not produce a high enough molecular weight when run with hydrogen and trialkyl boranes under conditions that allow the other component to produce low molecular weight polymer.